Fiber optic damage detection system for composite pressure vessels

ABSTRACT

An optical fiber wound onto an exterior surface of a composite structure in a two-dimensional pattern and adhered thereto to detect damage to the structure caused by impacts to or handling of the composite structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/781,162, filed Feb. 18, 2004, which is now U.S. Pat. No. ______,which is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to non-destructive testing methods and apparatusin general, and in particular, to the use of optical fibers as anindicator of damage to filament wound composite pressure vessels.

2. Related Art

A number of important fiber reinforced composite structures arecurrently being manufactured using filament winding. They are made in abroad range of sizes, and are categorized primarily by theirapplication. Aerospace applications include vessels for high-pressuregas containment, liquid propellant tanks, and solid rocket motors. Suchvessels are also used extensively in non-Aerospace applications, but arenormally designed to be much more robust, as weight control is typicallynot as critical. Currently, solid rocket motor cases of up to 5 feet indiameter are being made, and even larger sizes are possible. Theytypically have an elastomeric liner/insulator on the inside of the casewall.

Another class of composite pressure vessels are made for containment ofgasses at high pressures, and are usually relatively small, typicallyless than two feet in diameter. Those with thin metal liners are definedby the American Institute of Aeronautics and Astronautics (AIAA) inspecification S0-81 as “Composite Overwrapped Pressure Vessels,” or“COPVs”. There are also high pressure gas containment composite pressurevessels with non-metallic liners that have been used for commercialapplications. Very large liquid-containing propellant tanks have beenmade and these usually have a flexible liner on the composite wall forfuel containment. Typically, an epoxy-wetted fiber is wound in specifiedhoop and helical patterns on either a metallic liner, in the case ofCOPVs, or on a removable mandrel, and either with or without a liner atthe winding stage. Features common to all composite pressure vesselsused in aerospace applications are their relatively thin structuralwalls and thin metallic or elastomeric liners, which are used tominimize weight, and which render them relatively susceptible to shockand impact damage.

A preferred fiber for filament wound structures is carbon (graphite)fiber, which is exceptionally strong, but local deflections are limitedbefore breakage occurs. Surface impact events experienced by thesestructures can cause significant local deflections resulting in brokenfibers. The deflections can be elastic in nature, with no visualindication of subsurface fiber breakage or surface damage evident. Thereduction in strength of the composite structure resulting from brokenfibers can have implications that range from the need for relativelylow-cost repairs, to a catastrophic failure of the vessel that can leadto calamitous failure of a flight vehicle. Unfortunately, not all impactevents that occur subsequent to production inspection and that result inbroken fibers are visually observed and reported through appropriatechannels so that a detailed inspection is initiated to evaluate vesselintegrity.

The susceptibility to impact damage of composite pressure vessels can bemiti-gated somewhat through design by providing for adequate strengthafter visible impact damage has occurred and been confirmed by visualinspection. However, for some high performance composite structures thatare weight critical, such measures are not feasible, and multipleinspections are therefore necessary to ensure structural integrity.Accordingly, some space vehicle programs may expend very large sums ofmoney each year to re-inspect solid rocket motor cases for impact damageusing traditional non-destructive inspection methods prior to launch.Because of these high costs, new apparatus and methods for detectingimpact or handling damage in composite structures are needed.

Some efforts have been made in this area to decrease the time and costof traditional non-destructive inspections, including embedding sensingdevices into woven fiber cloth, or co-curing them between laminateplies. (See, e.g., U.S. Pat. No. 5,814,729 to Wu, et al.; U.S. Pat. No.5,245,180 to Sirkis; Claus, et al., “Nondestructive Evaluation ofComposite Materials by Pulsed Time Domain Methods in Imbedded OpticalFibers,” Review of Progress in Quantitative Nondestructive EvaluationVol. 5B, Thompson and Chimenti Eds., Plenum Press, 1986; Maslouhi, etal., “Use of Embedded Optical Fiber Sensors for Acoustic EmissionDetection Within Composite Materials,” 36th International SAMPESymposium, Apr. 15-18, 1991.) These references describe so-called “smartstructures” that are equipped with sensing devices that indicate when aply has been damaged. The drawback of these structures lies in theembedding within them of fiber optic sensors, which are typically largerthan the adjacent structural ply fibers, and which therefore canadversely affect the strength of the lay-up or sustain embedded fiberdamage during the curing process.

A need therefore exists for a low cost yet reliable method and apparatusfor detected shock, impact, handling and transportation damage tocomposite pressurized structures.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a reliable, low cost apparatusand method of use are provided for detecting structural damage incurredby a filament wound composite pressure vessel. In one advantageousembodiment, the damage detector comprises an optical fiber wound onto anexterior surface of the composite structure and adhered thereto with anadhesive. The core of the optical fiber may be silica, to approximatethe tensile strength and frangibility of the filaments of the underlyingcomposite structure. Advantageously, the optical fiber may be wound ontothe underlying structure while wetted with a coating of a liquid epoxyresin, and the resin then cured to adhere the fiber to the structure.The optical fiber is preferably wound on the composite structure in aregular, two-dimensional pattern in which adjacent windings are spacedat a selected distance from each other, and in which points along thelength of the fiber are mapped onto specific, known locations on thesurface of the underlying structure. The winding pattern may compriseeither helical or axial windings, or both.

Means are provided for transmitting a light signal though the opticalfiber, and for detecting the signal after its transmission therethrough.In one possible embodiment, a laser or a light emitting diode may beused to inject the light signal into an end of the optical fiber, and adiode, such as an avalanche photo diode, may be used to detect thetransmitted light signal. The light signal may be injected at a firstend of the optical fiber and detected at the opposite second endthereof, or alternatively, may be injected at a first end of the fiber,reflected back from the opposite second end, and then detected at thefirst end. Preferably, the light transmitter and detector areimplemented as small, portable units that are separate from the opticalfiber, but easily connectable to it in the field by optical connectors.

Immediately after the optical fiber is applied to the underlyingstructure, a first, or baseline, test is conducted with the apparatus todetermine a unique reference signal, or “signature,” of the undamagedoptical fiber. This reference or signature signal represents acompletely uninterrupted signal traveling at least once through thefiber. Then, after the composite structure has been subjected to theshocks, impacts and handling of transportation and/or installation, asecond signal similar to the first is sent though the optical fiberdetector in a manner similar to the first and compared with the originalreference or signature signal. Receipt of substantially the samebaseline reference signature signal from the device indicates that nointervening fiber breakage has occurred to the structure, whereas,substantial changes detected in the signal indicate that the structuremay have sustained structural damage that may not be obvious to visualinspection, but which could be detected by conventional non-destructivemethods, such as ultrasonic techniques.

In an alternative embodiment, a pulsed light signal is injected into oneend of the fiber, reflected back from the second end, and then detectedat the first end. Any discontinuity in the fiber occasioned by shock orimpact damage thereto will result in a reflected light signal that“leads,” i.e., is detected earlier than, the input signal reflected fromthe second end of the fiber. The amount of time taken by the reflectedlight signal to travel from the first end of the fiber to thediscontinuity and back to the first end is measured, and the distance ofthe discontinuity from the first end of the optical fiber then computedfrom the time taken. When this distance is located on the abovetwo-dimensional map of the optical fiber on the exterior surface of thecomposite structure, the location of the discontinuity, and hence, thelocation of potential damage to the underlying structure, is revealedprecisely.

A better understanding of the above and many other features andadvantages of the present invention may be obtained from a considerationof the detailed description thereof below, particularly if suchconsideration is made in conjunction with the several views of theappended drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing features and other features of the present invention willnow be described with reference to the drawings of a preferredembodiment. In the drawings, the same components have the same referencenumerals. The illustrated embodiment is intended to illustrate, but notto limit the invention. The drawings include the following Figures:

FIG. 1 is a perspective view of a liner or mandrel for a compositepressure vessel before being wrapped with a reinforcing fiber-and-epoxycomposite matrix;

FIG. 2 is a perspective view of the liner or mandrel of FIG. 1 afterbeing wrapped with a reinforcing fiber-and-epoxy matrix to form acomposite pressure vessel;

FIG. 3 is a perspective view of the composite pressure vessel after theapplication thereto of an exemplary embodiment of a damage detector inaccordance with the present invention,

FIG. 4 is a schematic diagram of an exemplary embodiment of a damagedetector in accordance with the present invention; and,

FIG. 5 is a schematic diagram of another exemplary embodiment of adamage detector in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A filament-wound composite structure 2, viz., a composite overwrappedpressure vessel, or COPV, to which the present invention hasadvantageous application, is illustrated in the perspective view of FIG.2. The particular composite pressure vessel illustrated comprises aliquid propellant tank, and includes a hollow, generally cylindrical,polymeric or metallic, e.g., plastic, titanium, aluminum or stainlesssteel, inner liner 4, such as that illustrated in FIG. 1. While theliner is shown as cylindrical in shape, it should be understood thatsuch a liner may have many other shapes, including, e.g., oblatespheroids, “near spheres,” or spheres. The underlying mandrel for thecomposite pressure vessel could also be a solid propellant casting or amechanically removable or dissolvable structure (e.g. salt or plaster)that is not an integral component of the final pressure vessel.

The exterior surface of the liner 4 may be densely over-wound in ahelical and/or longitudinal pattern with a reinforcing fiber, typicallya Kevlar (poly(p-phenyleneterephtalamide)) or Aramid, carbon and/orglass fiber, and coated with or embedded in a continuous layer of acured epoxy resin, which fills the interstices of the fiber windings andforms a strong, reinforcing matrix 6 that is initially supported by theliner, and which becomes an independent structural element after theepoxy binder is cured and an underlying mandrel removed. Winding can beaccomplished through either “wet-winding,” comprising dipping fibersinto a resin bath prior to wrapping them over the liner, or by “pre-pregwinding,” comprising winding a tape of fibers that have been previouslyimpregnated with resin onto the liner.

While the reinforcing fibers of the matrix have atensile-strength-to-weight ratio that is many times greater than that ofequivalently sized metallic fibers, they are also substantially morebrittle, and therefore, more susceptible to breakage from shear forcescaused by localized shock and impacts incurred by the structure duringmoving, handling or in-service abuse. To some extent, the resin bedserves to protect the fibers from such types of damage, but filamentwound composite structures nevertheless remain particularly susceptibleto damage caused by fiber breakage, and such damage is often hard todetect by a visual inspection of the structure.

A composite pressure vessel 2 having an exemplary embodiment of a damagedetector 10 applied thereto in accordance with the present invention isillustrated in the perspective view of FIG. 3. The vessel illustratedcomprises a cylindrical solid rocket motor case having an exhaust nozzle8 disposed at one end thereof. The damage detector comprises an opticalfiber 12 wound onto the exterior surface of the structure in an open,uniform, two-dimensional pattern in which adjacent windings are spacedat a selected, uniform distance from each other, and adhered thereto ina bed of a cured, i e., solidified, resin, e.g., an epoxy or apolyurethane resin.

In the particular exemplary embodiment illustrated in FIG. 3, thewinding pattern of the optical fiber 12 comprises overlapping helicalwindings 14 distributed along the long axis of the cylindrical motorcase 2 such that they define a grid pattern of, for example, 0.5 in,spacing when laid flat on a two-dimensional “map” of the exteriorsurface of the underlying structure, as illustrated in the inset detailof the figure. Other winding patterns, such as longitudinal windings,may also be used, but it is desirable that the windings be applied witha spacing, or area density, that is likely to be affected by localizedimpacts acting at points on the structure that are located betweenadjacent windings, as well as those which act directly on the opticalfiber itself. Additionally, it should be noted that, if the particularwinding pattern of the optical fiber is known, together with thelocation of the beginning and end points thereof on the above surfacemap of the underlying structure, then every point along the length ofthe fiber will correspond to a known location on that map, and can beused to locate impact damage to the structure precisely, as described inmore detail below.

The optical fiber 12 of the detector 10 may comprise either “plastic,”ie., polymer-based, or conventional “glass,” i.e., silica based, coreand cladding material, the latter so as to better approximate thetensile properties of the reinforcing fibers of the underlying structure2. The fiber may have, for example, a core diameter of between about 9to 63 microns (“μ”), and an outer diameter, including cladding and anybuffering, or reinforcing, layers, of about 125μ. Of course, the opticalfiber may also have other sizes and constituent parts, and in thisregard, it may be desirable to provide an optical fiber with minimal orno buffering layers to more closely mimic the frangibility of the fibersof the underlying structure, and to rely on a clear coating or paint, asdescribed below, for protection against minor scratches and abrasions.The fiber may also be, for example, either a “single-mode” or a“multimode” fiber, depending on the particular application at hand. Theformer type may be preferable as typically having a smaller diametercore, and therefore, being more sensitive to the level of impacts thatwould adversely affect the fibers of the underlying structure, whereas,the latter type may be preferable in some applications because of itstypically greater spectral “richness” caused by modal dispersion.

The optical fiber 12 may, like the underlying structure 2, be“wet-wound,” i.e., coated with an uncured liquid resin during winding,which is subsequently cured at an elevated temperature. However, theembedding resin is preferably cured at a lower temperature than that atwhich the underlying structure is cured, such that no loss in strengthwill occur in the underlying structure by the application to it of thedetector 10 . During the winding of the optical fiber, a first“pigtail,” or umbilical assembly 15 comprising a signal inlet/outletconnector 16, is adhered to the underlying structure to provide a signalinlet/outlet at a first end of the fiber, and in one possibleembodiment, a second umbilical 18 with either a signal outlet connectoror a reflector 20 may be disposed at the opposite, second end of thefiber, as illustrated in FIG. 3.

In another possible embodiment, shown in phantom in FIG. 3?, the secondend of the fiber can simply be terminated at a right angle so as to actas a reflector, in a known manner, and additionally, may be located awayfrom the first end, e.g., at the opposite end of the underlyingstructure 2. The umbilical arrangement may thus contain one or both endsof the optical fiber, as illustrated in FIG. 3, and may be applied atthe beginning of the wet winding procedure. Although the umbilicalassembly is adhered to the underlying structure, a majority of it maylie below the optical fiber after winding, thereby leaving only thesignal inlet, and optionally, outlet connectors exposed. A light, clearcoating of resin may be applied over the optical fiber/resin layer toprotect it against minor handling scratches, as discussed above, and ifdesired, the entire assembly can be painted over. After the final cureof the respective resins and coatings, the optical fiber damagedetection system is ready for use.

Immediately after the optical fiber 12 has been applied to theunderlying structure 2, it is desirable to perform a first, or baseline,test with the apparatus 10 to determine a reference signal, or“signature,” of the newly applied, undamaged optical fiber. Thisreference or signature signal represents a completely uninterruptedsignal traveling at least once through the entire length of the fiber.To effect this test, a transmitter unit (“TX”) 22, which may comprise aconventional light emitting diode (“LED”) or a laser diode, is coupledto the inlet connector 16 of the fiber, and a receiver unit (“RX”) 24,which may comprise a conventional PIN diode or an avalanche photodiode(“APD”), is coupled to the outlet connector 20 of the fiber, asillustrated schematically in FIG. 4. Alternatively, where the second endof the fiber comprises a signal reflector 20, both the transmitter andthe receiver may be coupled to the signal inlet/outlet connector 16 ofthe fiber, which may also include a signal coupler 26 to avoid the needfor an integrated transmitter/receiver device, as illustratedschematically in FIG. 5.

In the embodiment of FIG. 4, the light signal is injected into the firstend of the fiber, travels once through the length of the fiber, and isdetected at the second end of the fiber. In the embodiment of FIG. 5,the light signal is injected into the first end of the fiber, isreflected back from the second end of the fiber such that it travels thelength of the fiber twice, and is then detected at the first end of thefiber. In yet another possible embodiment (not illustrated), it ispossible to provide a “partial-reflector” at the second end of the fibersuch that a portion of the light signal is reflected back to the firstend of the fiber, while the remaining portion passes out the second endthereof. The two portions of the signal can then be compared with eachother, either before or after detection.

After the reference signal of the fiber is established, it is recordedand can be analyzed for amplitude or intensity, phase and spectralcontent. Then, after the composite structure 2 has been subjected to theshocks, impacts and handling of transportation and/or installation, asecond signal substantially similar to the reference signal is sentthrough the optical fiber detector 10 in a manner substantially similarto that in which the reference signal was sent and detected, and the twosignals then compared, which may include fast-fourier analysis, toassess potential intervening damage to the underlying structure. Thus,receipt of substantially the same baseline reference signal from thedevice indicates that no intervening fiber breakage has occurred to thestructure, whereas, substantial changes detected in the second signalindicate that the structure may have sustained structural damage whichmay not be visible to visual inspection, but which can be detected byconventional non-destructive methods, such as ultrasonic or x-raytechniques. For example, damage to the optical fiber which exposes evena small portion of the core of the fiber, or which results in a “kink”in the core, but which does not result in any discontinuity in the coreitself, will nevertheless result in a loss of in the second signal,which may be easily detected, and provide either a critical “go/no-go”indication, or alternatively, the need for the application of extensiveconventional non-destructive examination methods to the structure, suchas ultrasonic or x-ray techniques, to assess the location, nature andextent of the damage. Further, in a system with fiber optic connectorsat each end of the composite pressure vessel, a second signal can besent from the opposite end to verify that multiple damage locations donot exist beyond the initial finding that would require a completenon-destructive inspection of the structure to validate its over-allintegrity.

Where damage to the optical fiber 12 of the detector 10 results in adiscontinuity in the core of the fiber, at least a portion of a testsignal injected into the fiber will be reflected back from thediscontinuity to the first end of the fiber. In an embodiment of thedetector that detects a reflected signal at the first end of the fiber,such as that illustrated in FIG. 5, the signal reflected from thediscontinuity provides a mechanism for precisely locating the damage, inthe following maimer. In such an embodiment a pulsed light signal isinjected into one end of the fiber. The signal reflected back from thediscontinuity “leads,” ie., is detected earlier than, the input signalreflected from the second end of the fiber. The amount of time taken bythe reflected light signal to travel from the first end of the fiber tothe discontinuity and back to the first end is measured, and thedistance of the discontinuity from the first end of the optical fiber isthen computed from the time taken. When this distance is plotted on thetwo-dimensional map of the optical fiber on the exterior surface of thecomposite structure described above, the location of the discontinuity,and hence, the location of the potential damage to the underlyingstructure, is thereby revealed with substantial accuracy.

As will by now be evident to persons of skill in the art, manymodifications, substitutions and variations can be made in and to thematerials, configurations and methods of use of the reliable, low-costdamage detector 10 of the present invention without departing from itsspirit and scope. Accordingly, the scope of the present invention shouldnot be limited to the particular embodiments illustrated and describedherein, as they are merely exemplary in nature, but rather, should befully commensurate with that of the claims appended hereafter and theirfunctional equivalents.

1. Apparatus for detecting structural damage to a composite pressurevessel, the apparatus comprising: an optical fiber having a first endand a second end adhered to an exterior surface of the compositepressure vessel; an injector operable to inject a light signal into thefirst end of the optical fiber; and, a detector operable to detect areflected portion of the light signal at the first end of the opticalfiber; a reflector coupled at the second end of the optical fiberoperative to reflect a light signal injected into the first end of theoptical fiber back to the first end thereof; and means for measuring theamount of time taken by the reflected light signal to travel from thefirst end of the fiber to the reflector and back to the first end. 2.(canceled)
 3. The apparatus of claim 1, further comprising: a comparatoroperable to compare first and second light signals injected into theoptical fiber at different times.
 4. (canceled)
 5. The apparatus ofclaim 1, further comprising a fiber optic connector at the first end ofthe optical fiber.
 6. The apparatus of claim 1, wherein the light signalinjector further comprises a pulser operable to pulse the light signalinjected into the optical fiber.
 7. The apparatus of claim 1, whereinthe light signal injector comprises a laser or a light emitting diode.8. The apparatus of claim 1, wherein the detector comprises a PIN diodeor an avalanche photodiode.
 9. The apparatus of claim 1, wherein theoptical fiber includes a core comprising silica or a polymer.
 10. Theapparatus of claim 1, wherein the optical fiber is adhered to theexterior surface of the composite pressure vessel with a resin.
 11. Theapparatus of claim 10, wherein the optical fiber is embedded in theresin.
 12. The apparatus of claim 1, wherein the composite pressurevessel comprises a composite over-wrapped pressure vessel (“COPV”). 13.The apparatus of claim 1, wherein the composite pressure vesselcomprises a high pressure gas storage vessel, a liquid propellant tank,or a solid rocket motor case.
 14. (canceled)
 15. (canceled)
 16. Anapparatus for detecting structural damage to a filament wound compositepressure vessel, apparatus comprising: an optical fiber; means forinjecting first and second light signals into an end of the opticalfiber at different times; means for detecting a reflected light signalcorresponding to a discontinuity in the optical fiber; means formeasuring the amount of time taken by the reflected light signal totravel from the first end of the fiber to the discontinuity and back tothe first end; and means for computing the distance of the discontinuityfrom the first end of the optical fiber from the time taken.
 17. Theapparatus of claim 16, wherein the light signal comprises a pulsed lightsignal.
 18. (canceled)
 19. (canceled)
 20. (canceled)